1. Field of the Invention
This invention relates to a zoom lens for a television camera, and in particular to a zoom lens for a television camera which is very good in quality of image and is high in precision.
2. Related Background Art
Generally, an image pickup element for a television camera is particularly low in the sensitivity around the screen as compared with a film for a still camera and therefore requires the brightness of F-number 1.2 to 2. Particularly, as a zoom lens for a television camera, a zoom lens is required which maintains such brightness and yet has a large zooming area and has a zoom ratio of five to twenty times.
Numerous zoom lens types have heretofore been proposed, and a zoom lens of the type basically of the positive-negative-negative-positive four-unit construction has a merit that a relatively high magnification and a great aperture can be achieved. A lens system of this type is of four-unit construction in which a first lens unit is endowed with the focusing function, a second lens unit is endowed with the function of a variator for effecting magnification change, a third lens unit is endowed with the function of a compensator for correcting the fluctuation of the image surface caused by magnification change and keeping it constant and a fourth lens unit is endowed with the imaging function. A system is adopted in which when zooming is to be effected from the wide angle end to the telephoto end, the second lens unit is moved linearly toward the image side and the third lens unit is moved toward the object side so as to describe a convex curve.
Recently, the advance of the electronic image art is remarkable and television of high precision which is markedly better in quality of image than the conventional television images is being developed. As a lens for such a television camera of high precision, a lens of high performance having little residual aberrations is required. However, the conventional zoom lenses have great residual aberrations and also suffer from a great aberration fluctuation by zooming and therefore have been insufficient for use as lenses for television cameras of high precision.
In the conventional zoom lenses, particularly with zooming, the fluctuation of the curvature of image field from the intermediate position to the telephoto end (the telephoto position) has been very great and negative curvature of image field has been ready to occur at the telephoto end. If this aberration is corrected at the wide angle end and the telephoto end, positive curvature of image field occurs excessively at a position slightly toward the wide angle side from the telephoto end, and this is not preferable. Therefore, even if the power arrangement of each lens unit is loosened or the number of lenses is increased to improve the degree of freedom of aberration correction, it has been very difficult to correct the fluctuation of this curvature of image field. Therefore, in the conventional zoom lenses, it has not been possible to extract a satisfactory performance as a lens for high precision.